Acoustic wave devices using piezoelectric thin film resonators are used as filters of wireless devices for example. The piezoelectric thin film resonator has a structure in which a lower electrode and an upper electrode face each other across a piezoelectric film. Filters and duplexers are examples of the acoustic wave device using the piezoelectric thin film resonator. The piezoelectric film, lower electrode and upper electrode of the piezoelectric thin film resonator generally have negative temperature coefficients of elastic constants. This shifts a resonance frequency of the piezoelectric thin film resonator to a low frequency side with increase in temperature. As described above, the resonance frequency, anti-resonance frequency or frequencies of a passband vary with temperature in the above described acoustic wave devices.
There has been known a piezoelectric thin film resonator including a temperature compensation film inserted into a multilayered film to suppress a frequency change due to temperature, where the temperature compensation film is an insulating film such as a silicon oxide film having a temperature coefficient opposite to those of the piezoelectric film, lower electrode and upper electrode, as disclosed in Japanese Patent Application Publication No. 1-48694 and Proc. IEEE Ultrasonics Symposium 2009, pp 859-862.
There has been known a structure in which electrodes mutually short-circuited are located on upper and lower surfaces of a temperature compensation film to suppress a reduction in excitation efficiency caused by a concentration of electric field in the temperature compensation film when the temperature compensation film is used, as disclosed in Japanese Patent Application Publication No. 60-16010. There has been known a piezoelectric thin film resonator including a temperature compensation film embedded in the lower electrode, as disclosed in U.S. Patent Application Publication No. 2011/0266925.
However, the above described techniques improve temperature characteristics, including a resonance frequency, of the piezoelectric thin film resonator but degrade resonance characteristics including an electromechanical coupling coefficient. The temperature characteristics have a trade-off relationship with the resonance characteristics. This makes piezoelectric thin film resonators included in a single chip to have uniform temperature characteristics and resonance characteristics. Therefore, design flexibility is limited. For example, when resonators are formed so as to have insulating films of different thicknesses, the resonators can have different temperature characteristics and resonance characteristics. However, this increases the number of fabrication steps.
In addition, the effective electromechanical coupling coefficient decreases in the piezoelectric thin film resonator using the temperature compensation film when the temperature compensation film is thickened to improve the temperature characteristics. For example, the thickening of the temperature compensation film improves the temperature characteristics but degrades matching in a passband in a filter using the piezoelectric thin film resonator, or increases a loss within the passband. As described above, the temperature characteristics have a trade-off relationship with the effective electromechanical coupling coefficient. Therefore, it is difficult to control the temperature characteristics and electrical characteristics with respect to each of the piezoelectric thin film resonators in a single acoustic wave device.